How far from the lens (in cm) must the object be placed to accomplish this task, if the final image is located 20 cm from the lens?

Question

A 2.8 cm-tall object stands in front of a converging lens. It is desired that a virtual image 2.4 times larger than the object be formed by the lens. How far from the lens (in cm) must the object be placed to accomplish this task, if the final image is located 20 cm from the lens?

Answer 1

Hello Emma!

The magnification is 2.4.

M = - di / do.

If you use the sign convention that a virtual image indicates a negative di, then

2.4 = - (-20) / do

do = 20 / 2.4 = 8.33 cm

Hope that helps!